Histotripsy and Lithotripsy are non-invasive tissue ablation modalities that focus pulsed ultrasound from outside the body to a target tissue inside the body. Histotripsy mechanically damages tissue through cavitation of microbubbles, and Lithotripsy is typically used to fragment urinary stones with acoustic shockwaves.
Histotripsy is the mechanical disruption via acoustic cavitation of a target tissue volume or tissue embedded inclusion as part of a surgical or other therapeutic procedure. Histotripsy works best when a whole set of acoustic and transducer scan parameters controlling the spatial extent of periodic cavitation events are within a rather narrow range. Small changes in any of the parameters can result in discontinuation of the ongoing process.
Histotripsy requires high peak intensity acoustic pulses which in turn require large surface area focused transducers. These transducers are often very similar to the transducers used for Lithotripsy and often operate in the same frequency range. The primary difference is in how the devices are driven electrically.
Histotripsy pulses consist of a (usually) small number of cycles of a sinusoidal driving voltage whereas Lithotripsy is (most usually) driven by a single high voltage pulse with the transducer responding at its natural frequencies. Even though the Lithotripsy pulse is only one cycle, its negative pressure phase length is equal to or greater than the entire length of the Histotripsy pulse, lasting tens of microseconds. This negative pressure phase allows generation and continual growth of the bubbles, resulting in bubbles of sizes up to 1 mm. The Lithotripsy pulses use the mechanical stress produced by a shockwave and these 1 mm bubbles to cause tissue damage.
In comparison, each negative and positive cycle of a Histotripsy pulse grows and collapses the bubbles, and the next cycle repeats the same process. The maximal sizes of bubbles reach approximately tens to hundreds of microns. These micron size bubbles interact with a tissue surface to mechanically damage tissue.
In addition, Histotripsy delivers hundreds to thousands of pulses per second, i.e., 100-1 kHz pulse repetition frequency. Lithotripsy only works well within a narrow range of pulse repetition frequency (usually 0.5-1 Hz). Studies show that the efficacy and efficiency of lithotripsy decreases significantly when the pulse repetition frequency is increased to 10-100 Hz. The reduced efficiency is likely due to the increased number of mm size bubbles blocking the shock waves and other energy from reaching the stone.
Histotripsy transducers have a focal point positioned a distance from the transducer where the cavitational bubble clouds are formed. In order to non-invasively treat tissue inside a patient, the transducers must be positioned away from the patient's skin so as to locate the cavitational focal point on the target tissue. Thus, when the transducer is positioned away from the patient's skin, the pulsed ultrasound of a Histotripsy ultrasound transducer must be carried through an aqueous coupling medium that is in intimate contact with the ultrasound transducer and the skin surface.
One prior solution to acoustic coupling for therapeutic ultrasound includes a water bath disposed in a treatment table. During therapy, the patient lies with the body immersed in the water bath. This coupling solution is both cumbersome and expensive as it requires a specialized examination table and is not versatile or portable. Additionally, it requires a large volume of an acoustic coupling medium (typically degassed water) which is expensive and can be messy.
Thus, there is a need for an inexpensive, minimal, and versatile acoustic coupling device for use in ultrasonic therapy applications such as Histotripsy and Lithotripsy.